Automobiles travel by rotation of tires with a rotational drive force converted from a drive force that is generated by an internal combustion engine supplied with fuel. Recently, various attempts have been made to improve the fuel consumption ratio (gas mileage) of internal combustion engines on such automobiles, since the improved fuel consumption ratio reduces the amount of fuel that is consumed, thereby saving energy and protecting the global environment.
One such attempt is directed toward reducing resistance to sliding movement between the inner wall surfaces of cylinders (inner wall surfaces of bores or sleeves) of the internal combustion engine and the pistons that move back and forth in the cylinders. If resistance to sliding movement is reduced, then it becomes easier for the pistons to move back and forth in the cylinders. Therefore, the drive force applied to move the pistons back and forth is reduced, resulting in a reduction in the amount of fuel consumed.
It is known in the art to deposit a layer including a lubricant-rich material on inner wall surfaces of cylinders or piston skirts in order to reduce resistance to sliding movement and to improve the lubrication properties of the inner wall surfaces of the cylinders or the piston skirts. For example, in International Publication No. WO 2011/115152, the present applicant has proposed providing ridges on the sliding surface of a piston skirt and covering the ridges with a lubricating film made of silver, silver alloy, copper, or copper alloy.
As disclosed in International Publication No. WO 2011/115152, it is preferable to interpose an intermediate layer made of a heat-resistant resin material between the film and the piston skirt, so that the film becomes firmly bonded to the piston skirt by the intermediate layer. Specific examples of the heat-resistant resin material include polyimide resin, polyamide-imide resin, epoxy resin, nylon-6 resin, and nylon-6,6 resin, etc.
The existence of the film on the piston of the internal combustion engine is effective to maintain a lubricant suitably between the inner wall surface of the cylinder, e.g., the inner wall surface of the sleeve, and the piston skirt, and to diffuse or transfer frictional heat quickly, so that the piston skirt and the inner wall surface of the cylinder are prevented from becoming adhered to each other.